audio policy: fix AudioTrack output device selection

Have AudioTrack::setOutputDevice() set CBLK_INVALID flag instead
of calling restoreTrack_l(). This allows restoreTrack_l() to be called in
a safe context.

Allow device change while the AudioTrack is active by forcing a new
device selection in startOutput() if the output route for this
session was changed.

Remove some warnings.

Change-Id: I2d921a63c9bfa0e122233645e2d6d39f95f5f17d